Combustion chamber surfaces of an internal combustion engine

ABSTRACT

An arrangement in combustion chamber surfaces of an internal combustion engine, wherein these surfaces exhibit at least partially a thin surface layer (2) which will influence the octane/cetane rating requirement of the engine in a manner to limit the knocking tendency of the engine. The surface layer (2) comprises a material of low reflectivity and high absorptivity in respect of electromagnetic waves having a wavelength of up to at least 7 μm. The surace layer (2) has located therebeneath a layer (3) which functions as a transient heat buffer and alternately absorbs heat from and emits heat to the surface layer (2).

BACKGROUND OF THE INVENTION

The present invention relates to an arrangement of the combustionchamber surfaces of an internal combustion engine, in which thesesurfaces exhibit at least partially a thin surface layer which willinfluence the octane/cetane rating requirement of the engine so as tolimit the knocking tendency of the engine. An arrangement of this kindis proposed in Swedish Patent Specification No. 85 05 302-3, this knownarrangement being characterized in that when at least a part of thewalls of combustion chamber are treated in the manner prescribed, thewalls will absorb 90-95% of all thermal radiation in the energy-richwave length range of interest for influencing engine combustion.Furthermore, since the radiation which is reflected into the combustionchamber is diffuse, radiation from the walls of the combustion chambercontributes towards the occurrence of knocking in the combustion processto only a very slight extent.

As the energy-rich radiation is absorbed into the combustion chamberwalls it converts, however, to so-called joule's heat within the thinlayer of material, which is therewith heated rapidly to such hightemperatures as to eventually form a so-called "hot spot" during thecombustion process, which initiates knocking. Some of this radiation isalso reflected back into the combustion chamber as a result of the highsurface temperatures that prevail, resulting in heat loss.

The object of the present invention is to provide an improvedarrangement of the aforesaid kind in which the aforementioned drawbacksare fully or partially eliminated.

SUMMARY OF THE INVENTION

The invention is mainly characterized in that a heat buffer which has aspecial function is provided beneath said surface layer, and in that thenature of this surface layer is such that during combustion the layer isable to "capture" the energy-rich radiation and later, when thetemperature of the combustion chamber has fallen to a level beneath thelevel of the layer temperature, to emit radiation effectively to thecombustion chamber. The aforesaid special function of the heat buffer isto accumulate heat rapidly from the surface layer when the layerreceives heat from the combustion chamber, while at the same timecooling said surface layer, i.e. the heat buffer must be capable ofrapidly leading away heat and have a given thermal capacity. Subsequenthereto, it shall be possible to utilize as much of the stored heat as ispossible, to heat the surface layer upon the termination of thecombustion process. Cooling of the surface layer during the process ofcombustion counteracts knocking, while subsequent heating of saidsurface layer and the radiation of heat to the combustion chambermaintains the temperature level during expansion of the cumbustiongases, so as to obtain an improvement in efficiency. The simplest way ofachieving such a transient heat buffer is to incorporate beneath thesurface layer a layer of copper or silver having a thickness of about 1mm.

These metals conduct heat very rapidly, which is the most importantproperty expected of a good transient heat-buffer, i.e. there should bechosen a material which has a high value of thermal diffusivityaccording to the formula: ##EQU1## where λ=thermal conductivity,

ρ=density, and

c=specific heat capacity.

In order to utilize the material in the heat buffer to the best extentand to reduce the loss of heat therefrom to the engine cooling channels,the underlying layer is preferably arranged on a heat insulating layer,e.g. a thin layer of nickel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of the top of a piston according to one example ofthe invention, and

FIG. 2 is a diagram which illustrates temperature curves for two pointsin an internal combustion engine during a combustion cycle.

DETAILED DESCRIPTION

FIG. 1 illustrates the upper part of a piston 1 belonging to an internalcombustion engine. The piston may be one which has been cast from asuitable aluminium alloy in accordance with conventional techniques. Inaccordance with the invention, the illustrated upper piston partincorporates a surface layer 2 of black chromium (chromium oxides andchromium) having a thickness of about 4 μm, and a thicker copper layer3. The copper layer has a thickness of about 1 mm and is situated on athin layer of nickel 4, which forms a heat insulating layer.

The layers are conveniently applied electrolytically, subsequent to sandblasting the underlying surfaces. The copper layer 3 may be madeslightly thinner, particularly when it is situated on a nickel layer 4.

The effect of the arrangement of the present invention on thetemperature conditions t in a combustion chamber during the differentworking strokes (PS), compression I, expansion/combustion II, exhaustIII, and suction IV, is illustrated diagrammatically in FIG. 2. The fullline 10 in FIG. 2 indicates very schematically how the temperaturevaries in the combustion chamber during a combustion cycle. Thus, it isshown that the temperature will first rise slowly during the compressionstroke 10a and then rapidly to a peak value during the combustion 10b.The temperature will then fall rapidly during the final part of thecombustion process 10b and then at a slower rate during the exhauststroke 10c and suction stroke 10d.

The broken line curve 11 indicates the temperature of the surface layer2 and varies in time with the curve 10, although it has otheramplitudes. The surprising and significant fact about the curve 11 isthat the rise in temperature during the combustion process isinterrupted and that the temperature remains constant during a largepart of the combustion process and the exhaust stroke. In the case of aconventional piston, the temperature would follow the course of thechain line shown in FIG. 2. This is because the copper layer 3 locatedbeneath the surface layer 2 stores heat from the surface layer duringthe combustion process, therewith cooling said surface layer, anddelivers stored heat to the surface layer 2 upon completion of thecombustion process, therewith heating the surface layer 2 so as tomaintain or sustain the combustion chamber temperature during theexpansion of the combustion gases, during which the temperaturedecreases, thereby maintaining the pressure level and consequently alsothe engine torque in a more effective manner than was previously thecase. The emission properties of a conventional piston are inferiorunder such conditions. The fact that the surface layer 2 is heated bythe underlying layer in this way upon completion of the combustionprocess probably explains the reason for the marked reduction in carbondeposits observed when comparing an engine which incorporated theinventive arrangement with an engine which did not.

This comparison also showed that the emission of hydrocarbons was about30% lower in the engine which incorporated the inventive arrangement,which was surprising.

As will be understood, materials other than chromium oxide can be usedin the surface layer 2, for instance so-called cermet material, asdescribed in Swedish Patent Application No. 85 05 302-3, or quitegenerally such metal oxides as those used as selective absorber layersin solar energy collectors.

Furthermore, the cylinder head and valves may also be prepared inaccordance with the invention, either in addition to the piston head oralternatively thereto.

The arrangement of the invention is intended primarily for Otto-cycleengines, but may also be incorporated in diesel engines, since itdampens knocking in such engines quite considerably.

I claim:
 1. An arrangement in combustion chamber surfaces of an internalcombustion engine, comprising:a thin surface layer provided at leastpartially on the combustion chamber surfaces of the engine, and whichwill influence the octane/cetane rating requirement of the engine in amanner to limit the knocking tendency of the engine; said thin surfacelayer (2) comprising a material of low reflectivity and highabsorptivity in respect of electromagnetic waves having a wavelength ofup to at least 7 μm, and high emissivity at wavelengths longer than 7μm; and a further layer (3) located beneath said thin surface layer (2)for accumulating heat given off by said thin surface layer (2) during aprocess of fuel combustion in the engine so as to counteract a rise intemperature of said thin surface layer (2) and to heat said thin surfacelayer (2) during a part of the combustion cycle in which the temperaturein the combustion chamber is lower that the temperature of said thinsurface layer (2).
 2. The arrangement of claim 1, further comprising aheat insulating layer (4) underlying said further layer (3).
 3. Thearrangement of claim 2, wherein said heat insulating layer (4) is a thinlayer of nickel.
 4. The arrangement of claim 1, wherein:said thinsurface layer (2) comprises black chromium and has a thickness of about4 μm; and said further layer (3) comprises copper and has a thickness ofabout 1 mm.
 5. The arrangement of claim 4, further comprising a heatinsulating layer (4) underlying said further layer (3).
 6. Thearrangement of claim 5, wherein said heat insulating layer (4) is a thinlayer of nickel.
 7. The arrangement of claim 1, wherein:said thinsurface layer (2) comprises black chromium and has a thickness of about4 μm; and said further layer (3) comprises silver and has a thickness ofabout 1 mm.
 8. The arrangement of claim 7, further comprising a heatinsulating layer (4) underlying said further layer (3).
 9. Thearrangement of claim 8, wherein said heat insulating layer (4) is a thinlayer of nickel.
 10. The arrangement of claim 1, wherein said furtherlayer (3) is thicker than said thin surface layer (2).
 11. Thearrangement of claim 4, wherein said further layer (3) is thicker thansaid thin surface layer (2).
 12. The arrangement of claim 7, whereinsaid further layer (3) is thicker than said thin surface layer (2).